1) Field of the Invention
This invention relates to a gas turbine using air or a steam cooling system for cooling hot sections. More specifically, the present invention relates to an operation method of a gas turbine and a gas turbine combined electric generating plant which can execute a rated operation within a short period of time from the startup of the gas turbine and uses air or steam cooling with respect to hot sections, a computer program which realizes the operation method, and also relates to the gas turbine combined electric generating plant.
2) Description of the Related Art
To increase thermal efficiency in the gas turbine combined cycle, a technique in which steam is used as a coolant instead of air, to cool hot sections such as a dynamic blade and a stationary blade of the gas turbine with the steam, is now being used. The specific heat at constant pressure of dry steam is cp=1.86 kJ/kgK under a standard condition, which is a value almost twice as large as the specific heat at constant pressure of the air, cp=1.00 kJ/kgK. Therefore, the steam has a large heat capacity as compared with the air of the same mass, and the endothermic effect thereof increases. Further, if the wet steam is used as a coolant, latent heat of vaporization of the wet portion can be used for cooling, and hence the endothermic effect thereof further increases. Therefore, when the steam is used for the coolant, the cooling efficiency can be increased over the efficiency when using the air, and hence the temperature of the combustion gas at the entrance of the turbine can be set high. As a result, the thermal efficiency can be improved.
The air from the compressor has been conventionally used for cooling the dynamic and stationary blades of the turbine. However, if this compressed air is used for cooling, the work that can be taken out from the turbine decreases. Hence, if steam is used instead of the air, the cooling air for the dynamic and stationary blades can be saved, and the work that can be recovered by the turbine increases by this amount, whereby the generating efficiency can be increased.
FIG. 9 is a partial cross section of a gas turbine in which steam cooling is applied for dynamic and stationary blades. FIG. 10 is a schematic diagram showing a gas turbine combined electric generating plant adopting steam cooling for hot sections. In this gas turbine combined electric generating plant, thermal energy contained in the exhaust gas of the gas turbine is recovered by an HRSG (Heat Recovery Steam Generator) 370. Steam is generated by the thermal energy in the recovered exhaust gas of the gas turbine, and the high-temperature sand high-pressure steam is first supplied to a high pressure steam turbine 350 to drive it, to thereby generate power by the generator 355 coupled thereto.
The steam having worked in the high pressure steam turbine 350 is guided to a dynamic blade 321 through a steam supply pipe 311 provided in a turbine main spindle 310 of the gas turbine. Steam is also supplied to a stationary blade 325 from a steam supply port 330 provided outside of the casing of the gas turbine. A cooling flow passage is respectively provided in the dynamic blade 321 and the stationary blade 325, and the steam guided to the dynamic blade 321 and the stationary blade 325 absorbs heat of the combustion gas from the internal surface of the flow passage, while passing through this cooling flow passage, and is exhausted outside of the flow passage. Thereafter, the steam having cooled the dynamic blade 321 passes through a steam recovery pipe 312 provided in the turbine main spindle 310 and is taken out of the turbine, and the steam having cooled the stationary blade 325 is taken out of the turbine from a steam recovery port 331.
This cooling steam is guided to a mixing chamber 360 and mixed with the cooling steam having cooled the combustor tail pipe and the like, and the mixed steam is used as a working fluid for driving an intermediate pressure steam turbine 351 and a low pressure steam turbine 352. The steam having driven the intermediate pressure steam turbine 351 and the low pressure steam turbine 352 is recovered to water form by a steam condenser 365, and then supplied again to the HRSG 370 to repeat the above-described process.
In the gas turbine combined electric generating plant adopting the steam cooling system for hot members, hot members such as the dynamic blade and the stationary blade can be cooled effectively, and as a result, the temperature at the entrance of the turbine can be set high. Therefore, the thermal efficiency of the gas turbine is improved, and the thermal efficiency of the whole plant is also improved. However, in the gas turbine using the steam cooling system, it is necessary to change the coolant for the dynamic blade from the air to the steam, and to change the coolant for the stationary blade and the combustor tail pipe from the steam of the plant auxiliary steam generator to the steam of the HPSH (High Pressure Super Heater), during the period of from startup to rated operation. This is due to the following reasons.
The HRSG installed in the gas turbine combined electric generating plant has a large heat capacity, and hence it cannot generate the amount of steam necessary for driving the steam turbine and for using it as a coolant for the dynamic blade and the like for a while from the startup of the gas turbine. Therefore, it is necessary to use the steam supplied from a spare steam generator in the plant and the air in the gas turbine as the coolant until sufficient steam can be generated by the HRSG. When the HRSG can supply sufficient amount of steam, it is necessary to change the coolant for the hot members from the steam of the spare steam generator and the air in the gas turbine to the steam supplied from the HRSG.
For example, if the air for cooling the dynamic blade is changed at the same time when the gas turbine is connected with a generator, in some instances, a temperature distribution occurs in the gas turbine due to a density difference and a specific heat difference between the cooling air and the cooling steam, causing vibrations of the gas turbine, and thereby causing a trip (suspension) of the gas turbine. Therefore, to operate the gas turbine stably, the changeover of the coolant should be performed by finishing one operation and then executing the next operation by all means, and two or more operations are not performed at the same time. Hence, in the gas turbine combined electric generating plant using the steam cooling system, time is required from the startup to the rated operation of the gas turbine, whereby consumption of fuels and steam increases, causing a problem in that a starting loss increases. Further, if it is desired to obtain the rated output from a certain time, it is necessary to initiate the startup earlier, taking the time from the startup to the rated operation into consideration. As a result, extra personnel expenses for operating the plant are required.